1. Field of the Invention (Technical Field)
The present invention relates to methods for determination of parameters in lithography applications by diffraction signature analysis, including determination of center of focus in lithography applications, such as for photoresist lithographic wafer processing.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Lithography has a variety of useful applications in the semiconductor, optics and related industries. Lithography is used to manufacture semiconductor devices, such as integrated circuits created on wafers, as well as flat-panel displays, disk heads and the like. In one application, lithography is used to transmit a pattern on a mask or reticle to a resist layer on a substrate through spatially modulated light. The resist layer is then developed and the exposed pattern is either etched away (positive resist) or remains (negative resist) to form a three dimensional image pattern in the resist layer. However, other forms of lithography are employed in addition to photoresist litholography.
In one form of lithography, particularly used in the semiconductor industry, a wafer stepper is employed, which typically includes a reduction lens and illuminator, an excimer laser light source, a wafer stage, a reticle stage, wafer cassettes and an operator workstation. Modern stepper devices employ both positive and negative resist methods, and utilize either the original step-and-repeat format or a step-and-scan format, or both.
Exposure and focus determine the quality of the image pattern that is developed, such as in the resist layer utilizing photoresist lithography. Exposure determines the average energy of the image per unit area and is set by the illumination time and intensity. Focus determines the decrease in modulation relative to the in-focus image. Focus is set by the position of the surface of the resist layer relative to the focal plane of the imaging system.
Local variations of exposure and focus can be caused by variations in the resist layer thickness, substrate topography, as well as stepper focus drift. Because of possible variations in exposure and focus, image patterns generated through lithography require monitoring to determine if the patterns are within an acceptable tolerance range. Focus and exposure controls are particularly important where the lithographic process is being used to generate sub-micron lines.
A variety of methods and devices have been used to determine focus of stepper and similar lithography devices. Scanning electron microscopes (SEM) and similar devices are employed. However, while SEM metrology can resolve features on the order of 0.1 microns, the process is costly, requires a high vacuum chamber, is relatively slow in operation and is difficult to automate. Optical microscopes can be employed, but do not have the required resolving power for sub-micron structures. Other methods include the development of specialized targets and test masks, such as are disclosed in U.S. Pat. Nos. 5,712,707, 5,953,128, and 6,088,113. Overlay error methods are also known, as disclosed in U.S. Pat. No. 5,952,132. However, these methods, while increasing resolution because of the nature of the targets, still require use of SEM, optical microscopes or similar direct measurement devices.
A variety of scatterometer and related devices and measurements have been used for characterizing the microstructure of microelectronic and optoelectronic semiconductor materials, computer hard disks, optical disks, finely polished optical components, and other materials having lateral dimensions in the range of tens of microns to less than one-tenth micron. For example, the CDS200 Scatterometer, made and sold by Accent Optical Technologies, Inc. is a fully automated nondestructive critical dimension (CD) measurement and cross-section profile analysis system, partially disclosed in U.S. Pat. No. 5,703,692. This device can repeatably resolve critical dimensions of less than 1 nm while simultaneously determining the cross-sectional profile and performing a layer thickness assessment. This device monitors the intensity of a single diffraction order as a function of the angle of incidence of the illuminating light beam. The intensity variation of the 0th or specular order as well as higher diffraction orders from the sample can be monitored in this manner, and this provides information that is useful for determining the properties of the sample target which is illuminated. Because the process used to fabricate the sample target determines the properties of a sample target, the information is also useful as an indirect monitor of the process. This methodology is described in the literature of semiconductor processing. A number of methods and devices for scatterometer analysis are taught, including those set forth in U.S. Pat. Nos. 4,710,642, 5,164,790, 5,241,369, 5,703,692, 5,867,276, 5,889,593, 5,912,741, and 6,100,985.
Scatterometers and related devices can employ a variety of different methods of operation. In one method, a single, known wave-length source is used, and the incident angle "THgr" is varied over a determined continuous range. In another method, a number of laser beam sources are employed, optionally each at a different incident angle "THgr". In yet another method, an incident broad spectral light source is used, with the incident light illuminated from some range of wavelengths and the incident angle "THgr" optionally held constant. Variable phase light components are also known, utilizing optics and filters to produce a range of incident phases, with a detector for detecting the resulting diffracted phase. It is also possible to employ variable polarization state light components, utilizing optics and filters to vary the light polarization from the S to P components. It is also possible to adjust the incident angle over a range "PHgr", such that the light or other radiation source rotates about the target area, or alternatively the target is rotated relative to the light or other radiation source. Utilizing any of these various devices, and combinations or permutations thereof, it is possible and known to obtain a diffraction signature for a sample target.
Besides scatterometer devices, there are other devices and methods capable of determining the diffraction signatures at the 0th order or higher diffraction orders using a light-based source that can be reflected off of or transmitted through a diffraction grating, with the light captured by a detector. These other devices and methods include ellipsometers and reflectometers, in addition to scatterometers. It is further known that non-light-based diffraction signatures may be obtained, using other radiation sources as, for example, X-rays.
A variety of sample targets are known in the art. A simple and commonly used target is a diffraction grating, essentially a series of periodic lines, typically with a width to space ratio of between about 1:1 and 1:3, though other ratios are known. A typical diffraction grating, at for example a 1:3 ratio, would have a 100 nm line width and a 300 nm space, for a total pitch (width plus space) of 400 nm. The width and pitch is a function of the resolution of the lithographic process, and thus as lithographic processes permit smaller widths and pitches, the width and pitch may similarly be reduced. Diffraction techniques can be employed with any feasible width and pitch, including those substantially smaller than those now typically employed.
Diffraction gratings are typically dispersed, in a known pattern, within dies on a wafer. It is known in the art to employ multiple dies (or exposure fields) on a single wafer. Each diffraction pattern may be made by lithographic means to be at a different focus, such as by employing a different focus setting or a different exposure setting or dose. It is also known that center of focus may be determined using scatterometry and diffraction gratings by comparing diffraction signatures from a variety of different focus diffraction gratings to a theoretical model library of diffraction grating signatures yielding information regarding CD. The actual diffraction measures are compared to the model, from which CD values are derived. The CD value thus obtained is plotted against focus and the results fit to a parabolic curve. However, this method requires significant time and computer resources to generate the theoretical model.
The present invention provides a method of measuring parameters relating to a lithography device utilizing the steps of providing a substrate comprising a plurality of diffraction gratings formed on the substrate by lithographic process utilizing the lithography device, the diffraction gratings comprising a plurality of spaced elements; measuring a diffraction signature for at least three of the plurality of diffraction gratings by means of a radiation source-based tool; and determining the differences between the diffraction signatures to determine a desired parameter of said lithography device. In this method, the substrate can include a wafer.
The method can further include forming the plurality of diffraction gratings utilizing the lithography device at different known focus settings, and determining the two adjacent focus setting diffraction gratings wherein the difference between the diffraction signatures is less than the difference of the diffraction signatures between other adjacent focus setting diffraction gratings, whereby the parameter is the center of focus of the lithography device.
In a preferred embodiment, the different known focus settings are equal increment different focus settings. Alternatively, the different known focus settings are non-equal increment different focus settings, and the method further includes use of a mathematical algorithm to normalize the non-equal increment different focus settings.
The method further includes plotting the diffraction signature differences, wherein the difference in diffraction signatures between diffraction gratings increases as an approximation of a parabolic curve with a slope of zero over the center of focus. Determination of the difference in diffraction signatures between diffraction gratings can also include use of a metric. One metric that may be employed is a root mean square error method of data analysis. Determining the minimal difference can further include comparing the weighted averages of differences of diffraction signatures between diffraction gratings.
In one embodiment of the method, the method further includes forming a plurality of diffraction gratings utilizing the lithography device at the same focus setting and determining the differences as a function of the location of the diffraction gratings on the substrate. In another embodiment of the method, the method further includes forming the plurality of diffraction gratings at different known focus settings and different known dose settings and determining the effect of dose on focus. The plurality of diffraction gratings can include sets of the same known different focus setting diffraction gratings, the sets varying by different known dose settings.
The invention further provides a method of determining the center of focus in a lithography device, the method including the steps of providing a substrate comprising a plurality of diffraction gratings made utilizing the lithography device, the plurality of diffraction gratings comprising different known focus settings; determining a diffraction signature for at least three of the plurality of diffraction gratings by means of a radiation source-based tool; measuring the differences between the diffraction signatures between adjacent focus setting diffraction gratings; and determining the center of focus as the focus setting wherein there is a minimal difference between the diffraction signatures of adjacent focus setting diffraction gratings.
In one embodiment of this method, the difference in diffraction signatures between adjacent focus setting diffraction gratings increases as an approximation of a parabolic curve with a slope of zero on the minimal difference. Determining the difference in diffraction signatures between adjacent focus setting diffraction gratings can include determination of the difference using a metric, including but not limited to a root mean square error method of data analysis. The method also includes determining the minimal difference by comparing the weighted averages of differences between diffraction signatures of adjacent focus setting diffraction gratings. In yet another embodiment of this method, determining the minimal difference includes fitting data derived from differences between diffraction signatures between adjacent sequential focus setting diffraction gratings to a parabolic curve, whereby the minimal difference encompasses the minima of the parabolic curve.
In all of the foregoing methods, the radiation source-based tool includes light source-based tools. In one embodiment, the light source-based tool includes an incident laser beam source, an optical system focusing the laser beam and scanning through some range of incident angles, and a detector for detecting the resulting diffraction signature over the resulting measurement angles. The light source-based tool can further include an angle-resolved scatterometer. In a different embodiment, the light source-based tool includes a plurality of laser beam sources. In yet another embodiment, the light source-based tool includes an incident broad spectral light source, an optical system focusing the light and illuminating through some range of incident wavelengths, and a detector for detecting the resulting diffraction signature over the resulting measurement wavelengths. In yet another embodiment, the light source-based tool includes an incident light source, components for varying the amplitude and phase of the S and P polarizations, an optical system focusing the light and illuminating over some range of incident phases, and a detector for detecting the phase of the resulting diffraction signature.
In all of the foregoing methods, measuring a diffraction signature includes phase measurement by means of a broad spectral radiation source-based tool source, operating at a fixed angle, a variable angle "THgr" or a variable angle "PHgr". In the methods, measuring a diffraction signature also includes phase measurement by means of a single wavelength radiation source-based tool source, operating at a fixed angle, a variable angle "THgr" or a variable angle "PHgr". Measuring a diffraction signature can also include phase measurement by means of a multiple discrete wavelength radiation source-based tool source. The diffraction signature can be a reflective diffraction signature or a transmissive diffraction signature. The diffraction signature can be a specular order diffraction signature or a higher order diffraction signature, either positive or negative.
A primary object of the present invention is to provide a method for measuring parameters relating to a lithography device without the use of optical, SEM or similar microscopy metrology tools.
Another object of the present invention is to provide a method for determining center of focus of a lithography device by analyzing the diffraction signature difference between members of a series of different focus diffraction gratings.
Another object of the present invention is to provide a method for determining or measuring parameters associated with a lithography device, including center of focus, by obtaining a diffraction signature utilizing either reflective ortransmissive diffraction.
Another object of the present invention is to provide a method for determining or measuring parameters associated with a lithography device, including center of focus, by obtaining a diffraction signature utilizing any method to create a diffraction signature, including but not limited to reflective or transmissive angle-resolved, variable wavelength, variable phase, variable polarization state or variable orientation diffraction, or a combination thereof, of the 0th or specular diffraction order or any higher orders.
Another object of the present invention is to provide a method and device for determining or measuring parameters associated with a lithography device, including center of focus, without requiring direct use of either a theoretical model or library of known parameters.
Another object of the present invention is to provide a method for determining or measuring parameters associated with a lithography device, including center of focus, as a function of dose, by means of diffraction signature difference response and analysis.
Another object of the present invention is to provide a method for determining or measuring parameters associated with a lithography device by means of any order of diffraction signature of different focus diffraction gratings, including the 0th or specular order or any higher order diffraction, either positive or negative.
A primary advantage of the present invention is that it permits measuring parameters relating to a lithography device without the use of optical, SEM or similar microscopy metrology tools.
Another advantage of the present invention is that it permits use of a series of different focus diffraction gratings on a conventional wafer made by means of a stepper, including conventional photoresist lithography means, to determine center of focus utilizing determination of diffraction signatures, and the differences therebetween, for the diffraction gratings.
Another advantage of the present invention is that it provides a method and device that permits obtaining results, including center of focus, in a lithography device, such as a stepper, in a shorter period of time and at lower cost than conventional and known methods.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.